Let’s get this straight

THE ancient Egyptians were sticklers for detail. Despite their size, the pharaohs' tombs are positioned with extraordinary accuracy—the east and west walls of the Khufu pyramid, for example, are aligned north-south to within one-twentieth of a degree. Over the centuries, many competing theories have tried to explain how the pyramids' builders accomplished this feat of precision engineering. Was it luck, geometric prowess, or the helping tentacle of a visiting alien? Now an Egyptologist seems to have solved the riddle—by looking at how the ancients got it wrong, rather than how they got it right.

Using the heavens as a compass is complicated by precession—the slow swivelling of the earth's axis that causes the celestial north pole (the point in the heavens directly above the earth's north pole) to trace out a circle every 25,800 years. At the moment, the celestial north pole is near the star Polaris; in 13,000 years' time, it will be near the star Vega. In the night sky of ancient Egypt, though, no such quick-and-dirty placeholder was present. Kate Spence of Cambridge University suggests in this week's Nature that the pyramid builders must have used a couple of nearby stars to fix the location of the pole instead.

Dr Spence suggests that the Egyptians aligned the pyramids according to an imaginary line connecting two stars on opposite sides of the invisible pole—one in Ursa Major and one in Ursa Minor (familiar to Americans as the Big Dipper and Little Dipper). Using a plumb line, an ancient Egyptian astronomer could have determined when the line linking the two stars was vertical. The point at which the plumb line met the horizon would then indicate precisely the direction of true north.

Or at least it would for a couple of years either side of 2467BC. Before and after that date, this method would give an answer that was slightly wrong. But the error turns out to correspond closely with the observed errors in the pyramids' positions: the further their estimated dates of construction are from this window of accuracy, the larger the errors in their alignments. All of which strongly suggests that Dr Spence's method may indeed have been the one used to align the pyramids.

Furthermore, by working backwards from the alignment errors, this theory can be used to provide more accurate estimates of the dates of the pyramids' construction. Existing chronologies of ancient Egypt involve uncertainties of up to 100 years. But this new approach could narrow down that margin to just five years or so.

No doubt Dr Spence's work will disappoint some people—most notably, those who take the meticulous design of the pyramids as evidence either of extraterrestrial intelligence, or of the omniscience of the ancients. But, although this new hypothesis seems to demonstrate that the ancient Egyptians were unaware of the effects of precession, at least it restores credit for the design of the pyramids where it is due: they were built by ordinary men, using the stars as their guide.